The invention is directed to a method of manufacturing a glass composition which contains selenium with manganese compound for selenium retention and without the use of nitrate or nitrite. Preferably it includes a reductant like anthracite coal. The glass is a soda-lime-silica glass whose colorants consist essentially of iron oxide, selenium, manganese oxide, cobalt oxide, and optionally titanium oxide.
Many patents including selenium as a colorant have included sodium nitrate or potassium nitrate in the batch mixture to help improve the retention of selenium in the final product or for other purposes. For example, U.S. Pat. Nos. 3,296,004; 4,101,705; 4,104,076; and 4,190,452 all disclose bronze glass compositions using selenium together with sodium or potassium nitrates as components of their batches. U.S. Pat. No. 4,104,076 also teaches adding selenium and nitrates to the batch to make a gray glass composition as well as a bronze glass composition. U.S. Pat. No. 5,070,048 teaches a blue colored glass product using selenium together with sodium nitrate in the batch mixture. U.S. Pat. Nos. 4,339,541; 4,873,206; 5,023,210; 5,308,805; 5,346,867; 5,411,922; and 5,521,128 are all patents which teach using sodium or potassium nitrate in the batch when selenium is used as a colorant to make gray glass products. Hence, as seen from the above, it is extremely common in the glass making industry to include nitrates when using selenium as a colorant.
The present inventors have disclosed in U.S. Pat. No. 5,346,867 (Jones ""867) a gray glass whose colorants consist essentially of iron oxide, cobalt, selenium and manganese oxide and optionally titanium oxide, the manganese oxide providing selenium retention during processing. Batch materials disclosed therein include sodium and/or potassium nitrate to maintain oxidizing conditions early in the melting process which aids in selenium retention. The process for retarding volatilization of the selenium using a manganese containing compound along with the selenium during melt processing, which includes sodium nitrate as a batch component, is claimed in U.S. Pat. No. 5,521,128 (Jones et. al ""128), a divisional of the ""867 patent mentioned above.
One of the disadvantages of using nitrates or nitrites in batch processing is that they can generate environmentally undesirable nitrogen oxide emissions. In addition, we have found that when sodium nitrate is used in the batch, the amount of sodium sulfate (a fining agent) must be lowered which may adversely affect the fining action of sodium sulfate. Otherwise, the retention of selenium would be adversely impacted.
We have unexpectedly found that the manganese compound, e.g., used in the gray glass of Jones ""867 to retain selenium, also has sufficient oxidizing ability which allows nitrates and nitrites to be avoided throughout the manufacturing process of the selenium containing glass, contrary to what was formerly believed. This also allows the amount of sodium sulfate to be desirably increased over that which would normally be used when nitrates are included in the batch materials, which improves the fining action of the sulfates without negatively impacting the selenium retention. Hence, the present invention nitrate/nitrite-free manufacturing process provides a unique way to make a selenium containing glass, such as the ""867 Jones et al. gray glass, in a more environmentally friendly way while retaining selenium retention and in a way that overcomes problems which might develop based on the impact of using nitrates with sulfates.
In U.S. patent application Ser. No. 08/691,958 filed Aug. 5, 1996 and entitled xe2x80x9cReduction of Nickel Sulfide Stones In Glassxe2x80x9d, also having common inventorship and ownership with the present invention, a method is disclosed which involves using a manganese compound to prevent the formation of nickel sulfide stones by encouraging an oxidizing environment in the glass melt.
The present invention is an improved method for manufacturing a selenium containing glass, such as the particular soda-lime-silica gray glass of ""Jones 867, without including nitrates or nitrites in the glass batch during molten glass formation. And while excluding nitrates/nitrites, the method is able to maintain selenium retention as with nitrates or nitrates commonly included therein.
One preferred method is directed to manufacturing the gray composition of ""867 Jones which comprises by weight: 68 to 75% SiO2, 10 to 18% Na2O, 5 to 15% CaO, 0 to 10% MgO, 0 to 5% Al2O3, and 0 to 5% K2O, where CaO+MgO is 6 to 15% and Na2O+K2O is 10 to 20%; and colorants consisting essentially of: 0.9 to 1.9 wt. % total iron oxide as Fe2O3, 0.10 to 1.0 wt. % manganese oxide as MnO2; 0.002 to 0.025 wt. % cobalt oxide as Co, and 0.0010 to 0.0060 wt. % selenium as Se, and 0 to 1.0 wt. % titanium oxide as TiO2. The glass may also include tramp materials which sometimes enter the glass with raw materials or as a result of changeover of one glass composition to another in a glass furnace. For example, this would include up to about 0.005 wt. % nickel oxide as NiO.
The gray glass products made according to this preferred embodiment of the invention have the following spectral properties at 4.0 mm. thickness: 10 to 55% light transmittance using Illuminant A (LTA) and less than 25% ultra violet (UV) transmittance measured over the range of 280 to 400 nanometers and less than 50% infra red (IR) transmittance measured over the range of 720 to 2120 nanometers.
Further, this gray glass considered at a 4.0 mm. thickness preferably has the following spectral properties: 470-590 dominant wavelength and less than 5.5% purity of excitation.
The method comprises including a manganese compound while excluding nitrates or nitrites like sodium nitrate from the glass batch along with the selenium to retain selenium during melt processing. In order to maintain the same level of selenium retention with only manganese compound as would be retained with manganese compound and nitrate/nitrite, the level of manganese compound is increased in the batch components. That is, an amount of manganese compound needs to be added to replace the amount of nitrate/nitrate excluded. Excluding nitrates/nitrites from the manufacturing process of a glass composition which contains selenium while using manganese compound solely to retain selenium is contrary to what one would expect from the teachings in the patent literature.
Advantageously, we have unexpectedly found that the addition of a manganese compound alone, i.e., without any nitrate/nitrite compounds, to the glass batch to retain the selenium such as in the gray glass (Jones ""867 and ""128) is sufficient to retain the oxidizing conditions even at the beginning of melt processing so that no nitrates nor nitrites are necessary. That is, the manganese oxide acts in the glass batch to sufficiently shift the conditions of the glass batch toward oxidizing so that the selenium is retained without the use of the nitrates/nitrites, reducing undesirable nitrogen oxide emissions.
In addition, excluding nitrates/nitrites from batch processing of a glass containing selenium advantageously allows more sodium sulfate to be included in the melt for improved fining action in the glass batch during manufacturing.